Electromagnetic radiation in the THz frequency lies between the far infrared (which is generated by optical means) and microwave (which is generated by electrical means) regions of the electromagnetic spectrum. Because THz waves lie at the edge of the capabilities of both electrical and optical wave generation techniques, it was previously difficult, if not impossible, to generate THz waves needed for THz spectroscopy. Advances in the art have been able to produce radiation in the THz spectrum that could be used for THz spectroscopy. THz radiation is readily transmitted through most non-metallic and non-aqueous mediums, thus enabling THz systems to “see through” concealing barriers such as plastic packaging, corrugated cardboard, clothing, shoes, book bags, glass, etc. in order to probe the materials contained within (see, J. F. Federici et al., Semicond. Sci. Technol. 20 S266-S280 (2005); J. E. Bjarnason, et al., Appl. Phys. Lett., 85, 519 (2004)) while posing minimal or no risk to human health. Therefore, THz radiation is attractive for routine screening of people or animals. In addition, explosives and other dangerous agents have characteristic absorption spectra in the THz frequency range (see, W. R. Tribe et al., Proc. SPIE 5354 168 (2004); F. Huang et al., Appl. Phys. Lett., 85, 5535 (2004); H. Liu et al., Opt. Express 14, 415 (2006)), providing for THz waves a unique opportunity to distinguish these materials by their spectral signatures even if they are concealed behind barriers.
In 1995, Hu and Nuss demonstrated the first THz images. See, B. B. Hu and M. C. Nuss. Opt. Lett., 20 (16), 1716 (1995). Since then, THz imaging methods have been rapidly evolving due to advances in THz sources, detectors, and device fabrication methods. The simplest and most prevalent THz imaging method is the use of a single transmitter and detector pair wherein an image is obtained on a point-by-point basis by scanning the object through the THz beam which is focused by a parabolic mirror. Using this method, THz images of macroscopic objects have been obtained (see, Ja-Yu Lu et al., IEEE Photonics Technol. Lett., 17 (11), 2406-2408 (2005); T. Löffler et al., Appl. Phys. Lett., 90, 091111 (2007); I. S. Gregory et al., Appl. Phys. Lett. 86, 204104 (2005)) and extended to THz tomography. See, D. M. Mittleman et al., Opt. Lett. 22. 904 (1997).
Another approach to imaging known as THz synthetic aperture imaging has been investigated. Such methods require the THz amplitude and phase measured from multiple positions or from multiple beam paths to reconstruct the image. Synthetic phased array THz imaging uses arrayed optical mirrors to reconstruct diffraction-limited THz images. See, J. O'Hara and D. Grischkowsky, Opt. Lett. 27, 1070-1072 (2002). Image resolution can be improved when many individual images are superimposed.
Rapid THz spectroscopic data collection and image acquisition requires a faster scanning/modulation method.